A ratiometric converter is a converter whose output is inversely proportional to its reference voltage and directly proportional to its input voltage. That is: ##EQU1## Many transducer systems use the ratiometric converters to remove the drift and low frequency noise typically seen in the system's reference voltage. A typical transducer system includes a load cell that is powered by the reference voltage. The voltage difference developed across the load cell is proportional to the reference voltage multiplied by the forces applied to the transducer. This is input to the differential inputs of a ratiometric converter wherein the output is then inversely proportional to the reference that powers the load cell and also proportional to the output voltage of the transducer. By utilizing a ratiometric converter, the output does not depend on the system's reference voltage.
As a practical consequence, offsets that do not depend on the reference voltage, i.e., non-ratiometric, exist in the reference input to the ratiometric converter and also in the analog input sense lines. These offsets can be external or internal to the converter. Thermal emfs (thermocouples), charge injection and RF interference are some offset mechanisms. These offsets can introduce an error in the output that is dependent on the reference voltage. The measurement, therefore, is no longer ratiometric due to the presence of these non-ratiometric offsets, i.e., since they are independent of the reference voltage.
Present systems typically utilize AC excitation to remove external non-ratiometric offsets. In an AC excitation system, the system reference voltage is modulated, which in turn modulates the transducers output. Any non-ratiometric offsets that may exist in the reference voltage and in the input voltage sense lines are not modulated. Therefore, offsets that exist before the demodulation portion of the system are modulated up in frequency when the input and reference voltage signals are demodulated back down. These modulated offsets are removed by post-filtering. Any offsets that exist after the demodulation portion may still corrupt the ratiometric measurement. For low resolution systems, these offsets are insignificant because their effect is divided down by the instrumentation amplifier gain in the front end of the system. For high resolution systems, however, these offsets may still present a problem.
Another type of architecture, the "double pump" excitation architecture, provides a method for minimizing non-ratiometric offset effects. In this topology, the ratiometric digital output is a linear combination of two measurements; one for the reference voltage at half scale and the other for the reference voltage at full scale. This is analogous to an auto-zero scheme. The double-pumped architecture assumes that the two reference voltages are perfectly ratioed for cancellation of the non-ratiometric offsets. For example, if the ratio of half scale to full scale reference voltage is in error by one percent, then one percent of the non-ratiometric offsets is not canceled. Therefore, the double-pumped system still may have non-ratiometric offset errors that exist in the system.